Steam reforming catalyst

ABSTRACT

High activity sulfur tolerant steam reforming catalysts are described comprising iridium, palladium or platinum supported on lanthanum stabilized alumina or magnesium promoted lanthanum stabilized alumina. The catalysts have improved activity over conventionally used catalysts in the presence of sulfur containing hydrocarbon fuels, particularly in a steam reforming environment. The catalysts have utility in autothermal, tubular, cyclic and adiabatic steam reforming processes.

TECHNICAL FIELD

This is a continuation-in-part application of Ser. No. 857,688 filedApr. 30, 1986, now abandoned.

This invention relates to catalysts and more particularly to steamreforming catalysts for gaseous or liquid hydrocarbons.

BACKGROUND ART

Generally, catalytic production of hydrogen from hydrocarbon material isa two-step steam reforming process. A gaseous or liquid hydrocarbon feedstream is contacted with a catalyst and steam at high temperature,producing hydrogen, carbon monoxide, and carbon dioxide. These productsare then cooled and contacted with a shift conversion catalyst whichpromotes reaction of the carbon monoxide with steam, producingadditional hydrogen and carbon dioxide.

Prior to steam reforming, the hydrocarbon material is generallydesulfurized to prevent poisoning of the catalytic surfaces. While steamreforming can still be affected with the poisoned catalyst, catalyticactivity is reduced by several orders of magnitude. Generally, steamreformers are operate at higher temperatures to partially compensate forthis reduced activity. This significantly increases energy requirementswhile accelerating catalytic decay.

Various processes exist for desulfurizing a hydrocarbon material. Onedesulfurization process involves treating with hydrogen in the presenceof a hydrodesulfurization catalyst. This converts any sulfur in thehydrocarbon feed stream to hydrogen sulfide which is readily removed byadsorption on zinc oxide. However, such a process cannot be used todesulfurize heavier distillate fuels such as No. 2 fuel oil. Such fuelsare therefore not considered suitable fuels for steam reforming.

In commonly owned U.S. Pat. No. 4,414,140, issued to H. J. Setzer, highactivity sulfur tolerant steam reforming catalysts are describedcomprising rhodium or nickel supported on lanthanum stabilized aluminaor magnesium promoted lanthanum stabilized alumina. In commonly ownedU.S. Pat. No. 4,503,029, issued to H. J. Setzer, the improved steamreforming processes, utilizing the above catalysts, are described. Whilesuch catalysts have been successfully employed, the search continues forcatalysts which achieve even higher activities with improved sulfurtolerance.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a steam reformingcatalyst which is highly sulfur tolerant, providing efficient steamreforming of sulfur bearing hydrocarbon fuels, including No. 2 fuel oil.

It is a further object of the present invention to provide a steamreforming catalyst which maintains optimum conversion efficiency atminimized reformer operating temperatures, thereby maximizing energyefficiency with either gaseous or liquid hydrocarbon feed material.

These and other objects of the present invention are achieved byutilizing a catalyst comprising palladium, platinum, or iridiumsupported on a lanthanum stabilized alumina substrate. Such catalystshave been shown to maintain high activity with gaseous and liquidhydrocarbons, exhibiting high sulfur tolerance and increased life.

Another aspect of the invention comprises such catalysts supported on amagnesium promoted lanthanum stabilized alumina substrate.

Another aspect of the invention includes an autothermal reformingprocess utilizing the catalyst system according to the presentinvention.

Another aspect of this invention includes a tubular steam reformingprocess utilizing the catalyst system according to the presentinvention.

The foregoing, and other features and advantages of the presentinvention, will become more apparent from the following description andaccompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE illustrates the catalytic activities of the inventivecatalysts as compared to conventionally used steam reforming catalysts.

BEST MODE FOR CARRYING OUT THE INVENTION

The active catalytic components according to the present invention areiridium, palladium, and platinum deposited on a substrate supportmaterial. The choice of a substrate material is particularly importantto stability of the catalysts at elevated temperatures. For the iridium,palladium, and platinum catalyst systems, either a lanthanum stabilizedalumina or a magnesium promoted lanthanum stabilized alumina supportmaterial is used. The lanthanum stabilized alumina is a commerciallyavailable catalyst support material manufactured by W. R. Grace and Co.(Grace SRDX-1/79-1). The magnesium promoted lanthanum stabilized aluminais prepared by impregnating the lanthanum stabilized alumina with asolution (preferably aqueous) of a magnesium salt (preferably magnesiumnitrate) followed by drying to remove the solvent, and calcining in airto oxidize the deposited salt to magnesium oxide. Calcining temperaturesmay vary depending on the particular salt used, but generallytemperatures in the range of about 1800° F. (982° C.) are used, e.g. formagnesium nitrate. Enough magnesium salt is deposited on the supportmaterial such that after calcining about 3% to about 15% magnesium ispresent in the support material, and preferably about 5% by weight.

In addition to maintaining stability at elevated temperatures, thelanthanum stabilized alumina maintains a high BET(Brunauer-Emmett-Teller) surface area, dimensional stability andsufficient crush strength, especially when magnesium promoted. Forexample, the lanthanum stabilized alumina material reference abovemaintains a BET of about 90 at temperatures above 600° C. This substratematerial particularly promotes formation of small metal crystallites onthe surface which are necessary for catalytic performance, and hashigher resistance to carbon formation over other materials such asunmodified alumina.

The active catalytic material, either iridium, palladium, or platinum,is deposited on the substrate material by any conventional method.Generally, metal salts are dissolved in either aqueous or organicsolvents and dried on a substrate and then treated with hydrogen to formmetal crystallites. While metal deposition from the nitrates ispreferred, any acceptable route to form the metal crystallites on asubstrate material may be used, such as hydrogen reduction of the saltto form the metal crystallites or oxidation of the salt in air followedby reduction in hydrogen. The amounts of iridium, palladium, or platinumused may vary over a wide range, but are generally used in amounts basedon catalyst plus support material of 0.01% to 6.0% by weight platinum,0.5% to 15.0% palladium, and 0.01% to 6.0% iridium. Typically, amountsof 0.1% to 1.0% platinum or iridium, and 1.0% to 5.0% palladium arepreferred.

The inventive catalysts provide improved sulfur tolerance in tubularreformers, autothermal reformers, adiabatic reformers, and cyclicreformers. Such reformers vary in the manner in which heat is suppliedfor the endothermic reforming reaction. In a tubular reformer, such asthat disclosed in commonly assigned U.S. Pat. No. 4,098,589, which isincorporated by reference, the heat is supplied through the walls of acylinder to the catalyst material. In an autothermal reformer, such asthat disclosed in commonly assigned U.S. Pat. No. 3,976,507, which isincorporated by reference, the heat is supplied to the catalyst beddirectly by the heated gases entering the reformer. In a cyclicreformer, such as that disclosed in commonly assigned U.S. Pat. No.4,293,315, which is incorporated by reference, a plurality of reformersare operated simultaneously with one set of reformers, operating under acombustion phase (reacting fuel and air), providing the necessary heatfor the hydrogen production phase and the other set of reformers,operating under the hydrogen production phase, (reacting hydrocarbon andsteam), switching phases when the temperature of the reformers in thehydrogen production phase drops below that necessary to sustain hydrogenproduction. An adiabatic reformer utilizes a conventional heat exchangerto supply the requisite heat to the steam and hydrocarbon prior topassage into the steam reformer.

As stated above, in the autothermal reforming process fuel, steam andpreheated air are mixed and passed over the catalyst bed. The air isadded to the reactants to raise the temperature of the reactants andsupply the endothermic heat for reaction. In order to operateefficiently, the quantity of air added must be kept to a minimum. Arepresentative ratio of oxygen to carbon in the hydrocarbon is 0.35to 1. This tends to lower reaction temperature and increase the activityrequirements for any catalysts used in this environment. At operatingtemperatures, conventional steam reforming catalysts such as nickel onaplha alumina are deficient in activity and nickel on transition aluminalacks the surface area integrity and stability required for long termuse.

While iridium, palladium and platinum catalysts according to the presentinvention can be used alone, a particularly attractive arrangement forthe autothermal reformer includes the use of an inlet portion of ironoxide or other high temperature carbon tolerant catalyst in suchreformer. In this inlet region, all the oxygen reacts with thehydrocarbon and temperatures increase very rapidly. Downstream of thisregion, the reactor is loaded with the high activity iridium, palladiumor platinum catalyst of the present invention. In this latter region,hydrocarbon and reaction intermediates react with steam. Due to theendothermic nature of the reaction with steam, temperatures drop, and itis important to have a high activity catalyst in this region. The use ofsuch a multiple catalyst system allows greater flexibility in themaximum allowable reactor temperature and the method of introducing theair into the reactor.

Although the steam reforming catalyst according to the present inventionare not limited to fuel cell applications, when used for this purpose,sulfur containing fuels ranging from sulfur containing natural gas toheavier sulfur containing fuels such as No. 2 fuel oil can besuccessfully steam reformed. Synthetic fuels such as gasified coal andcoal derived liquids as well as hydrocarbons derived from sources otherthan petroleum, such as shale oil, are suitable for use with the presentinvention.

EXAMPLE 1

A lanthanum stabilized alumina catalyst support material was purchasedfrom W. R. Grace and Company in pellet form having dimensions of about0.318 centimeters diameter and about 0.318 centimeters length. Anaqueous solution is prepared by adding 0.221 grams of platinumdiaminonitrite to 12 milliliters water, then adding 8 milliliters ofconcentrated nitric acid. The lanthanum stabilized alumina pellets arethen immersed in the aqueous solution for five minutes with ultrasonicvibration and then 30 minutes without vibration. The pellets are thenremoved from the solution and dried in air for 3.5 hours at 110° C.

The treated pellets are placed in an oven which is alternativelyevacuated and filled with nitrogen three times. The temperature israised to approximately 316° C. and a gradual change in atmosphere fromnitrogen to hydrogen undertaken. (See Table 1)

                  TABLE 1                                                         ______________________________________                                                               Time in                                                % N.sub.2      % H.sub.2                                                                             Hours                                                  ______________________________________                                        100             0      0.25                                                   95              5      0.25                                                   90             10      0.25                                                   75             25      0.50                                                    0             100     2.00                                                   ______________________________________                                    

The pellets are cooled to 93° C. as the atmosphere is changed from 100%hydrogen to 100% nitrogen. The pellets are then further cooled to roomtemperature as the atmosphere is gradually adjusted to ambientconditions by the addition of oxygen. (See Table 2).

                  TABLE 2                                                         ______________________________________                                                               Time in                                                % N.sub.2      % O.sub.2                                                                             Hours                                                  ______________________________________                                        95              5      0.5                                                    90             10      0.5                                                    80             20      0.5                                                    ______________________________________                                    

EXAMPLE 2

Same procedure as Example 1 except the lanthanum stabilized aluminacatalyst support material is first immersed in an aqueous solution ofMg(NO₃)₃ ·6H₂ O having a concentration of 0.58 grams of magnesiumcompound per milliliter of solution prior to catalyst addition. Afterimmersion for approximately five minutes with ultrasonic vibration andthirty minutes without, the pellets are removed from the solution. Thepellets are then oven-dried in air for four hours at about 132° C. andthen calcined at 1036° C. for sixteen hours and cooled. Catalyst is thenadded as above described.

EXAMPLE 3

The following reactants were steam reformed in an isothermal tubularsteam reformer. Although only a micro reformer was employed having acapacity of 0.5 grams of catalyst material, the ratios used apply to anysize reformer.

    ______________________________________                                        Tubular Reformer (Isothermal)                                                 ______________________________________                                        Reactants                                                                     CH.sub.4 space velocity - 2.19 (g/hr)/g catalyst                              H.sub.2 O/CH.sub.4 ratio - 4.05                                               H.sub.2 /CH.sub.4 ratio - 0.365                                               H.sub.2 S concentrations - 2300 parts per million by weight                   Pressure - 1 atmosphere                                                       Platinum catalyst - 1.0 weight % platinum on                                  lanthanum stabilized alumina. Size 35-60 mesh.                                Temperature. °C.                                                                       687     700      715   726                                    % Conversion of hydro-                                                                        12.5    18.5     30.5  39.9                                   carbon to oxides of carbon                                                    Reaction rate constant (k)                                                                    0.29    0.46     0.80  1.11                                   Iridium catalyst - 1 wt. % iridium on lanthanum                               stabilized alumina. Size 35-60 mesh.                                          Temperature. °C.                                                                    727     740     758   774   784                                  % Conversion of                                                                            10.8    16.0    30.1  50.9  65.                                  hydrocarbon to oxides                                                         of carbon                                                                     Reaction rate                                                                              0.249   0.381   0.779 1.55  5.54                                 constant (k)                                                                  Palladium catalyst - 1 wt. % rhodium on lanthanum                             stabilized alumina. Size 35-60 mesh.                                          Temperature. °C.                                                                    755     781     819   851   882                                  % Conversion of                                                                            4.7     7.6     17.1  29.1  45.                                  hydrocarbon to oxides                                                         of carbon                                                                     Reaction rate                                                                              0.106   0.175   0.411 0.735 1.32                                 constant (k)                                                                  The reaction rate constant (k) (synonymous with                               activity) is defined by the pseudo-first order rate                           equation:                                                                      ##STR1##                                                                     ______________________________________                                    

In the Figure, the data for the catalysts is shown on a conventionalArrhenius graph. In this graph, the reaction rate constant k is plottedagainst the reciprocal of the absolute test temperatures. Forcomparative purposes, a plot of conventional 15% nickel catalyst onalpha alumina (A) is shown. Also shown are the improved nickel catalyst(B) and rhodium catalysts (C) of U.S. Pat. No. 4,414,140. In the abovetesting, a switch in sulfur contamination from hydrogen sulfide to suchcompounds as dimethyl sulfide, t-butylmercaptan, or tetrahydrothiophene,did not substantially affect catalyst performance. Changes in catalystloading on the substrate material from 0.1 to 6.0% by weight platinumhave shown a linear increase in rate constant up to about 1.0% to 1.5%by weight, at which point the curve flattens out.

From the figure, the order of activity proceeds from palladium (D) toiridium (E) to platinum (F) with all providing higher activity than thecommercially available catalyst (A). When compared to the improvednickel and rhodium catalysts, palladium is between while iridium andplatinum display significantly higher activity. While mixtures of thesematerials can be used, such mixtures do not impact any advantagesbecause of the phenomena of surface enrichment where the less activecatalytic material tends to migrate to the surface of the formedcomposite metal particles. While platinum is the most active catalyst asshown in the Figure, a number of factors must be weighted to determinethe most suitable catalyst for a particular application.

The platinum, iridium and palladium catalysts using the magnesiumpromoted lanthanum stablized alumina support are 15-20% less active thanthe platinum, iridium and palladium catalysts using lanthanum stablizedalumina, but retain their crush strength at higher temperatures than thelanthanum stablized alumina without the magnesium.

The catalytic material according to the present invention provides highactivity in a steam reforming environment with improved sulfurtolerance. These catalysts also allow reactors to be operated at lowertemperatures with greater efficiencies allowing less expensiveconstruction materials to be used. This provides particular advantagesfor adiabatic, autothermal and tubular steam reforming. While thesecatalysts have been described for use in steam reforming processesutilizing sulfur containing fuels, they could be used in conventionalsulfur-free steam reforming processes as well.

While this invention has been described in relation to steam reformingcatalysts used in fuel cell applications, it will be understood by thoseskilled in the art that various changes in terms of reformer design orcatalyst loading can be made without varying from the present invention.

We claim:
 1. A sulfur tolerant high activity steam reforming catalystconsistng essentially of platinum supported on a lanthanum stabilizedalumina which maintains a high BET surface area at temperatures above600° C. or a magnesium promoted lanthanum stabilized alumina substratematerial.
 2. The catalyst of claim 1 containing 0.01% to 6.0% by weightplatinum.
 3. The catalyst of claim 1 containing 0.1% to 1.0% by weightplatinum.
 4. The catalysts of claim 1 consisting essentially of platinumsupported on magnesium promoted lanthanum stabilized alumina.
 5. Asulfur tolerant high activity steam reforming catalyst consistingessentially of palladium supported on a lanthanum stabilized alumina ora magnesium promoted lanthanum stabilized alumina substrate material. 6.The catalyst of claim 5 containing 0.5% to 15.0% by weight palladium. 7.The catalyst of claim 4 containing 1.0% to 5.0% by weight palladium. 8.The catalyst of claim 5 consisting essentially of palladium supported onmagnesium promoted lanthanum stabilized alumina.
 9. A sulfur toleranthigh activity steam reforming catalyst consisting essentially of iridiumsupported on a lanthanum stabilized alumina or a magnesium promotedlanthanum stabilized alumina substrate material.
 10. A catalyst of claim9 containing 0.01% to 6.0% by weight iridium.
 11. The catalyst of claim9 containing 0.1% to 1.0% by weight iridium.
 12. The catalyst of claim 9consisting essentially of iridium supported on magnesium promotedlanthanum stabilized alumina.